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libavfilter/vf_colorspace.c: fix demarcation point of gamma linearize function

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The linearize function (usually refered to EOTF) is the inverse of
delinearize function (usually referred to OETF). Demarcation point of
EOTF should be beta*delta, but the actual value used now in the source
code is beta.

For ITU Rec.709, they are 0.081 (0.018*4.5) and 0.018 respectively
(beta = 0.018 and delta = 4.5), and they correspond to pixel value 5
and 21 for an 8-bit image. Linearized result of pixel within that range
(5-21) will be different, but this commit will make linearize function
of the filter more accurate in the mathematical sense.

Signed-off-by: Yonglin Luo <vincenluo@tencent.com>
Signed-off-by: Vittorio Giovara <vittorio.giovara@gmail.com>
This commit is contained in:
Yonglin Luo 2019-07-03 10:05:36 +08:00 committed by Vittorio Giovara
parent 1ac643f066
commit 664a27ea40
1 changed files with 2 additions and 2 deletions
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libavfilter/vf_colorspace.c
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@ -260,9 +260,9 @@ static int fill_gamma_table(ColorSpaceContext *s)
s->delin_lut[n] = av_clip_int16(lrint(d * 28672.0));
// linearize
if (v <= -in_beta) {
if (v <= -in_beta * in_delta) {
l = -pow((1.0 - in_alpha - v) * in_ialpha, in_igamma);
} else if (v < in_beta) {
} else if (v < in_beta * in_delta) {
l = v * in_idelta;
} else {
l = pow((v + in_alpha - 1.0) * in_ialpha, in_igamma);